AITA for not taking my girlfriend’s side after she embarrassed my friend?

Throwaway because my gf knows my main Reddit:

My gf (19F) Fiona and I (21M) have been together for around a month now, and yesterday I brought her to a Christmas party with all of my friends to introduce her to them. Fiona got along with all of them well, though she was understandably a little shy. It also make it difficult for her that most of my friends and I are international students whereas she’s born and raised in North America.

For context, Fiona doesn’t like to cause conflict and often is okay with letting things slide to keep the peace. However if she‘s angered, she’s also the type to say something and can be a bit of a hothead. That being said, she’s extremely sweet and understanding which makes this situation all the more bizarre for me.

Fiona was quiet most of the party, occasionally piping in the conversation and letting it take its course around her. She was holding a bag which I found out today is pretty expensive, around $2.7k. My friend Celia pointed out her purse and asked where she got it to which Fiona explained that it was a gift from her parents. At this, Celia asked what Fiona’s parents did.

Fiona told her that her mom was a stay at home mom and her dad worked in the oil field, and Celia made a face and said “you know digging oil is horrible for the environment.”

At this Fiona asked Celia if the clothes she was wearing was made of Polyester and Nylon, if she participated in fast fashion, if she took planes, or used solar powered energies primarily. And then she asked Celia if she drew the line for being environmentally conscious at someone working hard to support their family. Fiona’s tone was light but I could tell she was angry. The rest of my friends laughed and told Celia that Fiona had a point and moved on. But Celia went quiet afterwards and left soon after.

This morning, I got a text message from her that Fiona was extremely rude to her and that she didn’t feel comfortable attending parties where Fiona was present anymore and l that while she didn’t want to tell me what to do, that she thought I deserved someone better and less snobby.

When I told Fiona she went quiet and told me that the reason she snapped at Celia was because she was purposely acting as if Fiona didn’t exist and being snide towards her. I tried to argue that Celia never said anything off putting save for the “oil field” comment and that Fiona should just apologize to keep the peace but Fiona refuses and says I didn’t pick up on it because I’m a guy and girls have subtext. Now Fiona and Celia are both upset with me and I feel bad for putting them both in this situation. AITA for not picking Fiona’s side?

Update: we broke up, I’m dating Celia now.

Just kidding, I apologized to Fiona for not immediately standing up for her and showed her this thread. She’s been reading it for the past 2 hours laughing. She has NOT broken up with me, and I’ve sent a sternly worded text to Celia telling her that she was being rude and obviously attempting to pick a fight. Celia apparently tried to shit on Fiona to a couple friends, snidely remarking that she’s rude but they acted confused and told Celia she was mistaken. From what I picked up from conversations with friends, Celia was not in love with me as some people suggested, just jealous of Fiona. Also, it is normal in the country Celia is from to ask about parent’s occupation and Fiona said it wasn’t weird for her to ask about where the is purse from as she gets it a lot. I am deeply humbled and will now be throughly rethinking the way I behave in relation to my gf. Thanks 😭🙏